A rocket landed near a university in the Congolese city of Goma on Wednesday as thousands of civilians fled a fresh advance by M23 rebels that threatens to isolate the city of around two million people.

There were no casualties from the strike, which blasted a crater into open ground in the Lac Vert neighbourhood, but it underscored the potential threat to Goma, a strategic urban hub in the Democratic Republic of Congo's conflict-ridden east.

"This shows that M23 is targeting Goma now. The government has to do something to stop M23's progress," 25-year student Sophonie Bayonga said at the scene.

The M23 launched a major offensive in 2022, which led to military intervention and mediation efforts by East African regional leaders.

Congo, Western powers and a UN expert group have said the rebel group is supported by neighbouring Rwanda, which Rwanda denies.

Clashes between the rebels, army forces and self-defence groups that support them have escalated recently, forcing entire communities to flee to perceived areas of greater safety on the outskirts of Goma.

Jean-Pierre Lacroix, UN under-secretary-general for peace operations, described the situation around Goma as "very worrying" after an official visit to the province this week.

"It raises the risk of a regional explosion," he said on Wednesday, calling on the M23 to immediately cease its offensive and for diplomatic efforts to resume with all actors in the region.

He called for a reinforcement of Congolese forces in Ituri, North Kivu and South Kivu, the three provinces that a UN peacekeeping force is due to leave this year as part of its disengagement from the country.

Congo's government this week promised that it would not let Goma, which is situated on Lake Kivu close to the border with Rwanda, fall into M23 hands, as it did briefly in 2012.

The M23 said in a statement on Wednesday that this was not its goal and described its actions as "defensive manoeuvres".

Civilians have borne the brunt of the violence, with many killed in bombings and reprisal attacks.

Around 42,000 people have been displaced by the fighting since Friday, the UN's humanitarian office Ocha said on Tuesday.

Bombing and fire
Congo's army said on Wednesday that a second M23 bomb had hit a village about 15 km from the centre of Goma, between Tuesday night and Wednesday morning.

It added in a statement that its forces were fighting back attacks by the rebels around the nearby town of Sake.

The M23 made major advances last month that brought the conflict closer to Goma.

Natalia Torrent, head of a Doctors Without Borders (MSF) team in Mweso, about 100 km north of Goma, said clashes broke out two weeks ago as the army and pro-government militia tried to reclaim the town. Fighting picked up again over the weekend, wounding around 30 people, she said.

MSF has evacuated some of its staff after bullets struck a hospital in which thousands of Mweso residents were taking shelter. Most have since deserted the town.

The sound of bombing and gunfire rings out daily in both Sake and Goma, residents and Reuters reporters say.

"There was a lot of gunfire back home. The M23 burnt houses down and took everything," said Elisabeth Rebecca, who fled to Sake on Sunday from a nearby village.

"Some of us lost our children, there were many dead and many wounded," she said. Around her on the street, other displaced women cooked cornmeal over small open fires, surrounded by children.